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Dry and rainy season periods in the tank

by Kristian Adolfsson, MAF, [email protected].


Many fish from tropical areas have seasons when they spawn due to changes in their natural environment. Most often they spawn when the rainy season begins, because it brings increased food supply and increased possibility for the fry to find food and shelter. To try to re-create as many of the changes as possible that occur during the rainy season's beginning might be one way to spawn species that otherwise are very difficult to spawn. Many species are so easy to spawn you need not use these, in many cases cumbersome methods that are described below, but certain species and groups of species might need it. Begin with the general rules for breeding a certain species or group of species. If you don't succeed, try with the suggestions below.

The following is a compilation of a way to breed fish that come from areas with marked dry and rainy seasons i.e. the Amazon and Rio Negro areas in South America. Data and ideas have been collected from a lot of different sources, both from books, friends and the Internet, and are based on my own experience of breeding catfishes and tetras from South America.

This simulated dry and rainy season cycle takes about 4 weeks to go through. Using a simpler method people have been able to breed Panaque nigrolineatus, Sturisoma sp., and Siamese algae eater, which are thought to be very difficult to spawn.

Spawning triggers in nature

Below follows a list of the different changes that can occur during the rainy season's beginning and that might trigger a species to spawn. They are not listed in any particular order and which of them a certain species needs to spawn is not fully known.

  1. Low pressure
    After a long period of high pressure in the end of the dry season the barometric pressure falls in connection with the first rain.
  2. Increased food supply
    After a starvation period during the end of the dry season the food supply increases drastically. Certain species look like skeletons during the end of the dry season and have perhaps been without food for more than a month. Certain species even eat detritus to get some nutrition.
  3. Changed food types
    During the dry season the scarce food might consist of bottom dwelling animals (red mosquito larvae) and decaying plant parts. When the rainy season starts the food changes to insects that fall down on the surface, mosquito larvae (especially white and perhaps black) and other water insects, pollen from flowers, seeds, fruits, fresh leaves and eggs and fry from other species that have begun to spawn earlier.
  4. Increased water flow
    The rain results in increased flow of water. The fish have to get more active. Some species migrate up-stream to get to calmer and more suitable spawning areas.
  5. Increased oxygen levels
    The rain that falls on the water surface increases the oxygen level in the water. The increased water flow also makes the oxygen level increase. In many cases a high oxygen level is a condition for the eggs and fry to make it during their first days.
  6. Dilution of dissolved substances in the water
    The longer the dry season lasts, the more salts, humic substances and organic material will be concentrated in the amount of water that remains. When the rain starts the concentration of the above mentioned substances decrease in the water due to dilution. The river, the stream etc. is diluted with rainwater that has zero in hardness, which lowers the hardness and often even the pH.
  7. Change of the water temperature
    The water temperature is most often lowered due to cloudiness and the cold rainwater. How big the difference is, depends on from where the fish comes. In high terrain the temperature differences are most often larger than down in the lowlands (10

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